Gaddafi spy chief Abdullah al-Senussi held in Mauritania
Officials said Abdullah al-Senussi was detained at Nouakchott airport.
Mr Senussi, 63, was Gaddafi's brother-in-law, and has been described as one of his most trusted aides.
He fled Libya when Gaddafi was ousted and killed last year after an uprising and months of fighting. He is wanted by the International Criminal Court (ICC) for crimes against humanity.
Mauritanian security officials said Mr Senussi was arrested during the night as he arrived on a regular flight from the Moroccan city of Casablanca on a false Malian passport.
Neither the ICC nor the Libyan government have been unable to confirm the reports.
The BBC's Sebastian Usher says that if he is in custody he could provide the most detailed insights so far into the inner workings of the Gaddafi regime.
Uprising role
Correspondents say Mr Senussi, nicknamed "the butcher", is one of the last significant members of the regime still at large.
He was indicted by the ICC along with Gaddafi himself and the leader's son Saif al-Islam on 27 June 2011.
Saif al-Islam Gaddafi was captured in November in southern Libya and has been held by former rebels ever since.
Libyan, Arab and western sources describe Mr Senussi as a thuggish figure who would personally beat and abuse prisoners.
He was thought to have been responsible for purges of opponents within the regime in the 1980s and 90s, and for the deaths of 1,200 political prisoners at Tripoli's Abu Salim prison in 1996.
He kept a low public profile during last year's uprising, but reportedly played a key role in attempts to crush the revolt in the eastern city of Benghazi when it began last February.
There have been repeated reports of his death and capture which were later proved false.
Sources in the then opposition claimed he was killed in an attack by rebels in July in the Libyan capital Tripoli but later retracted the claim.
Nigerien officials said in October that he had fled through Niger into Mali, but a month later the new Libyan authorities said he had been arrested in the southern Libyan region of Sabha.
Further reports of his capture came in December but officials were unable to provide pictorial evidence.
